(1) Field of the Invention
The present invention relates of a control technique of an optical device to be utilized in various equipments for optical communication, in particular, to a control method and a control apparatus for controlling characteristics of optical device with high accuracy immediately after the starting of control operation.
(2) Related Art
There have been demanded large capacities and ultra-long distance of backbone networks with an explosive increase of data communication demand centered on Internet traffic. Further, at the same time, there have been demanded to realize the high reliable, flexible and economical networks, since the users require a great variety of services.
At present situations, the large capacities and ultra-long distance of optical networks have been remarkably developed by the wavelength-division multiplexing (WDM) transmission technique and optical amplification technique, thereby enabling to reduce the cost of transmission paths. However, in a case of increasing information processing ability of network node following the high-speed and large capacities of transmission signals, a conventional photoelectrical conversion and electric switch system results in an increase in node cost and a large scale. From the view point of these backgrounds, it is expected to develop an optical add and drop multiplexing (OADM) apparatus or an optical cross-connecting (OXC) apparatus that performs various processing in an optical wavelength region, by replacing large scaled electronic circuits for optical devices, in order to economize and miniaturize the node.
In these apparatuses, there are utilized many optical devices, such as, an optical switch having functions for switching ON/OFF of light, for attenuating the light, for switching to 1×n, or the like, an optical wavelength filter that separates a signal light for each wavelength, or the like. Among such optical devices as mentioned above, optical devices capable of collectively processing a desired plurality of wavelength lights for a WDM signal light are important key devices, in order to realize the OADM apparatus, the OXC apparatus and the like.
In the network node utilizing the OADM apparatus and the like including optical devices, such as, the optical wavelength filer or optical switch, for the purpose of flexibly managing networks, it is important to variably control characteristics of optical device corresponding to a desired plurality of wavelength lights, to enable to perform the collective process of arbitrary wavelengths for WDM signal light.
For example, in an optical wavelength filter that performs a process for collectively blocking optical signals of a plurality of wavelengths (blocking process) from passing through, a process for collectively dropping optical signals of a plurality of wavelengths (dropping process) and the like, it is required to control variable filter characteristics so that the center wavelength at a transmission band or blocking band is completely coincident with the wavelength of a desired optical signal to be selectively separated, to thereby enable to perform the collective process of arbitrary wavelengths for a WDM signal light. If the center wavelength in the optical wavelength filter does not coincide with the selected wavelength, in the blocking process, an extinction ratio is deteriorated or the passing of optical signal of other wavelength is erroneously blocked. Further, in the dropping process, an insertion loss at the node is increased or the optical signal of other wavelength is erroneously dropped. An occurrence of such situations should be avoided, since such situations become fatal for the processing operations of OADM apparatus or the like that constitutes network node.
Moreover, for example, in an optical switch that performs a switching process of optical paths for signal light, it is required to variably control with high accuracy switching element performance corresponding to a relation between input and output, to thereby enable to perform the accurate switching process of a plurality of optical paths. If the control accuracy in the optical switch is low, since, such as, an insertion loss at the switch is increased, an occurrence of such a situation should be avoided, as well as in the optical wavelength filter.
As a method to variably control characteristics of the optical device according to a desired relationship to be set corresponding to signal light to be processed in the above manner (for example, the relationship of selected wavelength, optical path or the like), it can be considered a method to monitor an operation state of the optical device, to thereby control the operation setting of the optical device based on the monitoring result. Specifically, in a case where the characteristics of the optical device are changed by an influence of the deterioration due to time lapse, an environment change, a control error or the like, or the wavelengths of the optical signals to be input to the optical device is fluctuated by unstable wavelengths in a transmission light source, it is possible to monitor a processing result at the optical device, to thereby feedback control a control parameter in accordance with the monitoring result.
When the consideration is made on such a feedback control of the operation setting of the optical device as mentioned above, however, there is a problem in that it is difficult to reliably obtain the characteristics of the optical device according to a desired relationship. Namely, if the monitoring result at the starting of control is different from that obtained from a desired characteristic, a certain control period of time is required until the characteristics of the optical device reach an allowable range by the feedback control of control parameter. Therefore, it is difficult to ensure the characteristics of the optical device during this period of time. In the optical devices to be utilized in the OADM apparatus or the OXC apparatus, if the process of blocking the passing of desired wavelength light, dropping desired wavelength light, switching the optical paths or the like is erroneously performed, the services to the user shall be suspended. Therefore, the operation control of the optical device requires to be at high accuracy from the initial condition thereof. In order to satisfy such a demand, a control technique of optical device that dissolve the problems in the feedback control as mentioned above is indispensably required.